It is believed that chiral symmetry, which is spontaneously broken in our world, should be restored in a system with a finite temperature and/or a finite density. Theoretically, it has been suggested that a signature of the chiral symmetry restoration could be seen as a possible mass shift of vector mesons in nuclear matter.This research project has aimed the mass of ρ, ω and φ mesons produced, and decays in nuclei, through a determination of the invariant mass from the e^+e^- pairs, or the K^+K^- pairs following their decays. The experiment was successful to observe ω and φ mesons in the e^+e^- invariant mass spectra, and reported the signature of an in-medium mass modification of the mesons. This was the first observation in the leptonic in-medium decay of vector mesons at a normal nuclear matter density. The experiment finished the data taking in March 2002, and the statistics is improved by factor 100 compared to the previous publication and the preliminary data have been reported in international conferences. In another half year, we expect to reach the final results.